The present invention relates to a signed document transmission system which transmits digital data of a document, along with digital data indicating a signature attached thereto, and permits the receiving side to verify the validity of the signature and the document.
In transmitting a document in the form of digital data, it is relatively easy to partly or wholly falsify the document, and/or to change the signer thereto, and in addition, it is generally impossible for the receiving side to learn such changes.
In view of the above, such a system has been proposed that processes digital data for transmission on the transmitting side and permits the receiving side to verify the validity of the received document and signature in the form of digital data. This system is commonly referred to as a signed document transmission system.
Known conventional signed document transmission systems include one that employs common ciphers, represented by DES (Data Encryption Standard) (S. M. Matyas, "Digital Signatures--An Overview", Computer Networks, Vol. 3, No. 2, pp 87-94, 1978) and one that employs public keys, represented by the so-called RSA method (R. L. Rivest, "A Method for Obtaining Digital Signatures and Public-Key Cryptosystems", Communication on ACM, Vol. 2, No. 2, pp 120-126, 1978). The former system using common ciphers possesses the defects of complexity in the procedures for signature generation and verification and a large amount of data to be stored on the side of verification. On the other hand, the latter system using public keys (hereinafter referred to as the RSA method) is the most typical one in terms of safety and simplicity of procedures but has the drawback of low processing speed. To get around this problem, a system employing a hashing technique (hereinafter referred to as the hybrid system) has also been proposed (D. W. Davies, "Applying the RSA Digital Signature to Electronic Mail", IEEE COMPUTER Fed., pp 55-62, 1983), but this system encounters difficulty in increasing the processing speed for a signature to a short document.